This invention is generally directed to processes for the preparation of liquid developer compositions, especially liquid developers wherein there is selected a low vapor pressure component that is entrapped in the developer. More specifically, the present invention is directed to a process for the preparation of liquid developer compositions which comprises entrapping a low vapor pressure component, for example about 0.01 millimeter of mercury, in a resin material. Thus, in one embodiment of the present invention the process involved comprises the preparation of resin particles containing a pigment in the low vapor pressure component, and thereafter adding thereto pigment particles and charge directing additives enabling liquid developers wherein the low vapor pressure component is entrapped within the resin preventing its undesirable evaporation, or release into the environment after fusing of a developed electrostatic image. In a further embodiment of the present invention, there is provided a process for the preparation of liquid ink compositions which comprises heating a resin component to above its melting point in a low vapor pressure oil, which heating is usually accomplished at a temperature of from about 80.degree. to about 130.degree. C.; thereafter dispersing the pigments such as carbon black in the resulting heterogeneous oil/resin mixture; and cooling the dispersion to room temperature, which is usually accomplished over a period of from about 4 to about 6 hours while simultaneously attriting this mixture resulting in particles of a suitable average diameter, usually from about 2 to about 3 microns in a preferred embodiment of the present invention. The aforementioned process enables both the pigment and the oil to be entrapped within the resin particles. Also, the dispersion formed is added to a low viscosity oil, such as Isopar, together with the charge director to enable the formation of an electrophoretic liquid developer composition. Accordingly, upon fusing the formed liquid developer composition in an imaging or printing apparatus the low vapor pressure oil present is permanently trapped within the resin thereby eliminating any odor which might emanate from the imaging or printing apparatus as is the situation, for example, when a high vapor pressure or volatile oil is selected for the formation of the developer compositions. With further respect to the formed liquid developers, they are referred to as electrophoretic developers; according, it is necessary to disperse them in a low viscosity oil to enable them to migrate to the latent electrostatic image within short time periods, that is within milliseconds. Therefore, as the mobility of the ink is believed to be directly related to the viscosity of the dispersion medium, a low viscosity oil, for example from 1 to about 3 centipoises, is selected as the dispersion medium for the developer particles. The liquid inks of the present invention can be selected for the development of images in various processes, inclusive of xerographic processes, electrostatic printing, and facsimile systems; color proofing processes; and the process as illustrated in Savin British Patent Publication No. 2,169,416, published July 9, 1986, the disclosure of which is totally incorporated herein by reference.
Development of electrostatic latent images with liquid developer compositions commprised of, for example, a dispersion of pigments in a liquid hydrocarbon are known. In these methods, the electostatic latent image, which is usually formulated on a photoconductive member, is transported through a bath of the aforementioned liquid developer. Contact with the liquid developer causes the charged pigment particles present therein to migrate through the liquid to the photoreceptor surface in the configuration of a charged image. Thereafter, the toner electrostatic image is then electrostatically transferred from the photoconductor surface to plain paper. The image transfer efficiency from the photoreceptor to the paper is typically about 50 to 60 percent since the phenomenon of ink splitting between the photoreceptor and the paper usually occurs. In this process, the excess liquid toner remaining on the photoreceptor has to be cleaned therefrom prior to the next image cycle, and the excess toner discarded. Also, as only about 50 to 60 percent of the liquid toner is transferred to paper, the optical density of the image is often unacceptably low. For example, with the compositions and processes illustrated in U.S. Pat. No. 4,476,210 the reflection optical density of black liquid toner images is typically of from 1.0 to 1.1 while dry developers typically exhibit values of 1.2 to 1.4. Further, liquid toners exhibiting transfer efficiencies greater than about 80 percent also provide dense black images of optical density greater than 1.2, thus minimizing the amount of toner that has to be discarded and rendering an effective cleaning of the photoreceptor surface. The thin film of residual developer remaining on the surface of the sheet is then evaporated within a relatively short time period, usually less than 5 seconds. Also, the making pigment particles may be fixed to the sheet by heat, for example, in image configuration.
There are disclosed in U.S. Pat. No. 3,554,946 liquid developers for electrophotography comprised of a carrier liquid consisting of a hydrocarbon, negatively electrostatically charged toner particles dispersed in the carrier liquid, and a pigment therein such as carbon black, aniline black, prussion blue, phthalocyanine red, and cadmium yellow. In accordance with the teachings of this patent, a copolymer is coated on the surface of the pigment particles for the primary purpose of imparting a negative electrostatic charge to these particles. Other patents disclosing similar liquid developer compositions include U.S. Pat. Nos. 3,623,986; 3,625,897; 3,900,412; 3,976,583; 4,081,391 and 3,900,412. In U.S. Pat. No. 3,900,412, there is specifically disclosed a stable developer comprised of a polymer core with a steric barrier attached to the surface of the polymer selected. In column 15 of this patent, there are disclosed colored liquid developers by selecting pigments or dyes, and physically dispersing them by ball milling or high shear mixing. Attempts to obtain color liquid developer compositions by the ball milling process described have been unsuccessful, particularly with respect to obtaining developed images of acceptable optical density in that, for example, the desired size for the latex particles is from 0.2 to 0.3 micron in diameter; and with ball milling techniques it is very difficult to provide a dispersion of carbon black or other pigment particles much smaller in size than about 0.7 to 0.8 micron. Consequently, the addition of carbon black pigment particles, for example to latex particles with a diameter of 0.2 to 0.3 micron, result after ball milling in relatively small latex particles residing on the surface of the pigment particles.
Additionally, there is described in U.S. Pat. No. 4,476,210, the disclosure of which is totally incorporated herein by reference, liquid developers containing an insulating liquid dispersion medium with marking particles therein, which particles are comprised of a thermoplastic resin core substantially insoluble in the dispersion, an amphipathic block or graft copolymeric stabilizer irreversibly chemically, or physically anchored to the thermoplastic resin core, and a colored dye imbibed in the thermoplastic resin core. The history and evolution of liquid developers is provided in U.S. Pat. No. 4,476,210, reference columns 1 and 2 thereof.
Furthermore, there is illustrated in British Patent Publication No. 2,169,416, the disclosure of which is totally incorporated herein by reference, liquid developer compositions comprising toner particles associated with a pigment dispersed in a nonpolar liquid, and wherein the toner particles are formulated with a plurality of fibers or tendrils from a thermoplastic polymer, and possess a charge opposite to the polarity of the latent image. These toners apparently permit in some instances excellent transfer efficiencies, however, they have been found to be difficult to prepare, for example, it has been found difficult to obtain the same result from batch-to-batch with this mechanical process. Also, the time needed to prepare such particles of the order of up to 30 hours, which is uneconomical. Furthermore, since the preferred resin, for example Elvax II 5720, is Isopar insoluble, it entraps Isopar during the mechanical grinding process. Consequently, during the fusing step energy has to be expended in driving Isopar out of the resin as well as fixing it to the paper surface, which enables as an energy inefficient liquid toner.
Other prior art of interest include U.S. Pats. Nos. 4,454,215 and 4,058,774 directed to liquid developers, and more specifically suitable toner particles present in a specific liquid carrier while considering the composition of the toner particles so as to form readily disassociated flocs. reference column 4, lines 63 to 68, and continuing on to column 5, lines 1 to 14; U.S. Pat. Nos. 3,954,640; 4,024,292; and 4,268,597, which disclose liquid developers with vehicles of Magie Oil 520 or 620. Patents primarily of background interest are U.S. Pat. Nos. 4,306,009; 4,363,863; 4,374,918 and 4,521,505. The main disadvantage associated with the aforementioned prior art inks resides in the small particle size.
Although the above described liquid inks, and processes for the preparation thereof are suitable for their intended purposes, there remains a need for new processes. More specifically, there is a need for processes for the preparation of liquid developers wherein the vehicle component is entrapped, and thus cannot be released as is the situation with some prior art inks. There also is a need for the preparation of colored liquid developers which possess the aforementioned characteristics. Additionally, there is a need for the preparation of liquid developer compositions that permit images of excellent resolution in a number of known imaging processes. Moreover, there is a need for liquid developer processes wherein the colorants selected are suitably dispersed in a resin such that the primary particles are preferably of an average diameter of from about 2 to about 3 microns, thus enabling black or colored images of excellent resolution. Further, there remains a need for liquid developer processes wherein there are included in the final compositions other additives, inclusive of charge directors. There is also a need for processes for the preparation of liquid developers with entrapped vehicles thereby permitting a reduction in the amount of energy needed, about 25 percent for example, for fusing; and where long term copy odor problems are avoided. Additionally there is a need for processes for liquid developers with low vapor pressure entrapped components, such as Magiesols, enabling a reduction in the amount of this component that is released to the environment.